Well here goes... Aluminium frame router design

[CharlieRam]

i also am wondering about that diagonals and at the same time big holes in the middle. I don't know the top you are planning on but generally i wouldn't have a hole in my table bigger than 300mm, otherwise you will need 50-70mm top if made from ply or similar. even alu top would benefit from such maximum distance.

I will add another cross beam, maybe 2 then if I have enough material, as I mentioned earlier the plan is to put 4" x 1" 3mm alu box section over the length of the bed and then 10mm alu plate on top of that drilled and tapped.

IMHO you are mistaken about the ball screw end mounts. Do yourself a favor and design it similar like i did them from 10mm plate and additional plates that screw to that perpendicular reinforced with ribs plate.
Or you could find your self in a scenario that you have to file shims or even worse, what you will do if the ball screw has to go inside direction?

I understand what you are saying but so long as the rail seating faces are parallel to the bearing block seating faces and the bearing blocks are machined correctly then I will only have to adjust the distance to the Z plate which I am hoping to adjust with the ballnut housing bracket, out of interest how accurate are the ballscrew end blocks from base to centre height? If the heights are out I can always grind them at work while no one is looking!
Cheers, Charlie